[CodeCompletion] Fix issue causing the completion status to not be set correctly for pattern completion

We weren’t setting the code completion token status correctly when parsing patterns with code completion tokens.

Because of this, in the added test case, the `searchSubject` gets added as a member of `Foo` twice - once in the first pass and once in the second pass, causing an assertion failure.

Fixes rdar://80575116 [SR-14687]

lib/Parse/ParsePattern.cpp

@@ -1138,7 +1138,7 @@ ParserResult<Pattern> Parser::parsePattern() {
       // code complete anything here -- we expect an identifier.
       consumeToken(tok::code_complete);
     }
-    return nullptr;
+    return makeParserCodeCompletionStatus();
     
   case tok::kw_var:
   case tok::kw_let: {